The GOP repeal-and-replace plan faces daunting hurdles on the other side of the Capitol. Using a budgetary process known as reconciliation, Republicans need 50 votes in the Senate to pass the measure. And while the party holds 52 seats, those numbers will be tough to come by. GOP moderates such as Susan Collins of Maine have criticized the House bill for rolling back coverage, conservatives like Kentucky’s Rand Paul have slammed the measure for leaving Obamacare’s infrastructure in place and up-for-election senators like Jeff Flake of Arizona may balk at the prospect of kicking millions of Americans off their health coverage. It’s unlikely a single Democrat would side with Republicans to unravel a major piece of President Obama’s legacy.

“It will be a real big challenge on the Senate side,” said Republican Senate Leader Mitch McConnell. All of which makes McConnell the most powerful legislator in Washington right now. The Kentuckian is considered among the smartest tacticians in either party, and there is little doubt that he is capable of devising a strategy to thread the measure through the Senate.

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The big question is whether he will, and under what conditions. After the House hurdle, McConnell said cryptically in a statement that “Congress will continue to act on legislation to provide more choices and freedom in health care decisions.” He has tasked Sen. John Thune of South Dakota with heading up an ad hoc committee to rewrite the House version of the bill. McConnell and his deputy, Sen. John Cornyn of Texas, have told colleagues that they won’t bring the bill to the floor if it doesn’t have the votes. It’s McConnell’s way of insulating them from casting votes that could be held against them during their re-election campaigns.

In the lobbying shops on K Street and the marble corridors of the Capitol, members of both parties have been whispering for months that McConnell may actually have little interest in repealing Obamacare outright. Yes, McConnell rails against the Affordable Care Act with the best of ‘em, and no, it doesn’t mean he secretly likes Obama’s signature legislation. But McConnell, 75, is as pure a political animal as anyone in town. And unlike House Speaker Paul Ryan, who faced the possible prospect of a right-flank revolt or an angry Trump tweet if he failed to move the bill through the lower chamber, McConnell has the luxury—and the burden—of balancing his party’s desire to replace Obamacare with a complicated political equation.

Polls show Obamacare is increasingly popular, and the Ryan plan far less so. An earlier estimate indicated 24 million Americans could lose their health care coverage. It’s still unclear how it will cover or hike costs for people with pre-existing conditions, who won a blanket non-discrimination clause under Obamacare.

Ending federal programs that help millions of Americans has never been a winning play, and Democrats are ready to pounce. They taunted their opponents as House Republicans filed out of the chamber toward the buses waiting to take the GOP to the White House. “You’re walking the plank for what? A bill that will not be accepted by the United States Senate,” House Democratic Leader Nancy Pelosi warned Republicans in a last-minute plea from the House floor. “You will glow in the dark on this one.”

McConnell wants to shield his members from taking perilous votes. He has no particular desire for his party to shoulder the blame for every kink in a sprawling, complex segment of the economy. And unlike Ryan, McConnell wants to hear the costs from the Congressional Budget Office before he schedules a vote.

Meanwhile, McConnell also has to consider his constituents. Back in Kentucky, roughly a third of the state’s residents are on Medicaid—a program that expanded under Obamacare. Because of Obamacare, the uninsured rate dropped to less than 7%, lower than the national rate of 11%. Its state-run health care exchange, Kynect, is often held up as a model of a program executed exceptionally well. Moves to undercut it would dog McConnell at home.

McConnell has proved his ability to resist outsiders’ demands. Just look at the Senate’s refusal to even consider Obama’s nomination of Judge Merrick Garland to fill a Supreme Court vacancy. One way he could do so is simply to continue to insist that Republicans don’t have the votes. Wily party leaders often use this as a pretext to refuse to take up legislation that might inflict long-term political damage. Take Democratic Senate boss Harry Reid’s unwillingness to bring up House-passed cap-and-trade legislation when his party held the House, Senate and White House for the first two years of Obama’s first term.

Arcane Senate procedures, which McConnell knows well, can also give him an out to scramble the entire House package. In some cases, parliamentary requirements and budgetary laws actually compel McConnell to make major changes to what the House sent over—such as allowing states to ask permission to offer plans that lack, say, prescription drug benefits or drug and alcohol treatment. Senators have been privately frustrated that Ryan made promises to factions in his party that won’t survive the Senate scalpel. “He knows better,” said one GOP Senator, who added sympathetically the Speaker was trying to navigate the tricky politics of his fractious conference.

There are reasons for McConnell to seek those changes. The bill’s cost is unknown; the Congressional Budget Office did not have time to estimate its price tag. The Wall Street Journalwarned that a loophole in the bill would leave workers whose health care coverage comes through their employers on the hook for more out-of-pocket costs. Yes, the cost of buying insurance might go down. But so, too, would benefits.

Even Republicans know the House version was merely the first draft, albeit one that took several miscues to complete and will be weaponized against incumbent lawmakers in the 2018 midterm elections. Pieces of the measure, including provisions that would cut off funding for states that wanted to expand Medicaid programs, were unlikely to survive the final package that Senate renegotiates. The ping-pong of messages volleyed back and forth between House and Senate leaders included more than a few winks and nods that this whole process resets itself once the 126-page measure makes its way north across Capitol Hill. The bill, Republican Sen. Lindsey Graham of South Carolina tweeted, “has not been scored, amendments not allowed, and 3 hours final debate—should be viewed with caution.”

For their part, the House was simply relieved to move the bill off their plate. “The Senate is a legislative body. They can do what they want with the bill. Then we will go to conference,” Kevin McCarthy, the No. 2 Republican in the House, told CNN ahead of the vote. Even as he was cajoling colleagues to support the measure, he was nodding that the House GOP version was just the jumping-off point, not a final version.

None of this, however, seemed to catch with the President. Trump delayed his trip to New York to meet with the Australian Prime Minister so that he could welcome victorious House members to the Rose Garden. It was a bill signing without a bill because, of course, the Senate hadn’t even had its first read of what the House did. “Our friends over in the Senate,” Ryan joked at the post-vote White House rally, “are eager to get to work.” Lawmakers and White House officials laughed heartily.